1. Field of the Invention
The present invention relates to a mask for intercepting light for a lens and more precisely, it relates to a light intercepting mask which prevents ghost or flare or the like produced by a specific reflecting surface of a lens of a photographing lens unit consisting of groups of lenses.
2. Description of Related Art
A photographing lens unit in a recent camera is compact but complicated to increase the zoom power (magnification), resulting in a complex internal construction of the lens barrel. For instance, in a lens arrangement shown in FIG. 4 in which a group of rear lenses (rear lens group L2) is constituted by a meniscus lens having a concave surface on the side thereof adjacent to an object to be taken, incident light through the upper periphery of a group of front lenses (front lens group L1) is first reflected by a front surface B (on the object side) of a third lens of the rear lens group L2 closest to a film F and is then reflected by a rear surface C (on the film side) of a second lens (intermediate lens) of the rear lens group L2 to be focused on a point D. An image of the point D may be exposed and formed as a ghost E on the film F.
In the lens system shown in FIG. 4, the front surface B of the third lens of the rear lens group L2 defines a specific reflecting surface by which the ghost is produced. No ghost can be completely prevented by an aperture frame G which is located in rear of the specific reflecting surface to define an image plane.
Furthermore, in a normal photograph, the film plane is rectangular with a longer horizontal side, whereas the photographing lens is circular in cross section. Consequently, in particular, incident light (luminous flux) through the upper and lower edges of the photographing lens includes a large amount of harmful components, thus resulting in a production of a ghost.
It is known to provide a hood in front of the photographing lens in order to eliminate the ghost.
However, a conventional hood has a circular cross sectional shape corresponding to the front shape of the photographing lens, and accordingly, there is a difference in shape between the image plane and the front portion of the lens. In particular, in a zoom lens, since an angle of view (focal length) varies, it is very difficult to realize a simple hood which can effectively prevent the ghost through all the angles of view.
Note that in the present application, a lens surface which causes an image to be damaged (e.g. by a ghost) is referred to as a specific reflecting surface.